(1) Field of the Invention
The invention relates to a control system for controlling a rotorcraft rotor. Such a control system is adapted to enable the collective pitch and/or the cyclic pitch of the blades of a main rotor or of a tail rotor to be modified, and in particular to modify the angle of inclination of the plane of a main rotor as defined at each instant by the rotary motion of the free ends of the blades of the rotor. This angle of inclination serves to modify the direction of the lift force produced by a main rotor and consequently enables a rotorcraft to take up its various specific flight configurations.
The invention also relates to a rotorcraft fitted with such a rotor control system and to a method of controlling a rotor by means of a piloting member. In addition, below in the present application, the term “piloting member” is used to designate any control device suitable for being operated manually by a pilot or automatically, whether inside a cockpit or outside the rotorcraft, when the rotorcraft does not carry a pilot. Such a piloting member may have various forms and in particular it may be in the form of a cyclic pitch lever or a collective pitch stick for controlling the pitch of the blades of the rotor, i.e. the aerodynamic angle of incidence of the blades, or it may be a mini-stick or “joystick” for controlling the cyclic pitch or the collective pitch, or indeed it may be a set of pedals for modifying the pitch angle of the blades of a rotorcraft tail rotor.
(2) Description of Related Art
In conventional manner, rotorcraft control systems have piloting members with large lever arms and/or large amounts of travel between two physical abutments defining the travel stroke of a piloting member. Under certain circumstances, the stroke of the free end of a conventional collective pitch stick can be very large, and for example it may be more than several tens of centimeters.
Furthermore, with such conventional piloting members, it is usual practice to have a single piloting control that is proportional in position. Under such conditions, a position of the collective pitch stick corresponds always to the same position for the rotor plane of the rotorcraft, for example.
Such a mode of operation is used in particular when a mechanical control system or the piloting member and the rotor are connected together mechanically by a linkage made up of cranks, connecting rods, and swashplates.
Under such circumstances, a large amount of travel for the collective stick makes it possible to cover the entire control range of the rotor and enables the rotorcraft to be piloted with good sensitivity. For each movement of the collective pitch stick, there is a proportional movement of the members of the rotor and the resulting response of the rotorcraft is controllable.
Furthermore, the large amount of travel of the collective pitch stick also serves to provide a lever arm that is sufficient to counter large control forces.
Nevertheless, even with a large amount of travel, holding the stick in a determined position can be difficult to achieve or uncomfortable. That is why an assistance device, generally referred to as a “trim box”, enables the pilot to place the collective pitch stick in a given position and leave it to means suitable for supplying a compensating force to hold the collective pitch stick in this position. In the jargon of rotorcraft piloting, this function is sometimes referred to as “anchoring the stick” or indeed “cancelling forces” (on the stick, understood).
Furthermore, in order to further limit the forces in the piloting member and improve piloting comfort, electrical or optical controls have subsequently been developed that enable the various movements of a rotor to be controlled. Under such circumstances, a computer receives information representative of a position of the collective pitch stick, of the cyclic pitch lever, or of the pedals, and in turn it controls at least one servo-control proportionally. The at least one servo-control thus enables the movements of the rotor to be controlled. Such an electrical or optical control system is thus advantageous in that it enables the piloting member to be decoupled mechanically from the rotor that it controls.
In addition, in order to simplify equipment, a piloting member, such as a joystick in particular, may also be passive. Under such circumstances, it does not have any motor means enabling its position to be further controlled. Such a joystick thus returns to its neutral force position if the pilot no longer exerts a position-holding force on it.
Nevertheless, under such circumstances, it can be difficult for the pilot to know whether the current position of a servo-control controlled by the joystick is close to a limit of its control range. Specifically, the piloting member may be positioned in a central position of its stroke, corresponding to a “force neutral” position, but this position may be completely decorrelated from the setpoint transmitted to the servo-control for controlling the movement of the rotor. Thus, the control associated with the current position of the servo-control, referred to in this application below more generally as the “control”, may itself be close to a limit.
Furthermore, Document EP 2 258 616 describes a method and a system for controlling and regulating the drive of a hybrid helicopter. It also describes the possibility of limiting the propulsion of a rotor when the nose-down margin drops below a predefined threshold.
Nevertheless, in that Document EP 2 258 616, nothing indicates how the operating relationships for controlling propulsion is modified by a control unit.
Document EP 2 821 341 describes means for taking over the controls of an aircraft that was initially using an auto-pilot system.
Nevertheless, such a document does not describe modifying an operating relationship for the control of a rotor as a function of variation in a current control margin for controlling movements of a rotor.
Finally, Document EP 2 457 792 describes a vehicle piloting device that represents the technological background of the invention and that therefore does not make provision for modifying an operating relationship for controlling a rotor.